Inclusion body myositis (IBM) is the most common acquired muscle disease occurring over the age of 50, with an annual incidence estimated at 2-5:100,000. Although it was initially thought to be a variant of polymyositis, multiple clinical and pathologic studies have clearly defined it as a specific entity, distinct from the two other common idiopathic inflamatory myopathies: dermatolmyositis and polymyositis. IBM may either be immunologically mediated or immune attack of muscle may play a role in muscle damage. Since the immune-altering regimens tried thus far have been unsuccessful in altering the clinical course or the immunohistologic findings in muscle, other immune-modulating strategies must be considered. Inferferon beta 1a is effective in multiple sclerosis, another disease in which the effect of conventional antiinflammatory agents produce brief major benefit but at most, equivocal long-term improvement in course.